The present invention relates to an improvement of a steering force control apparatus including a hydraulic reaction mechanism suitable for a speed response type power steering system.
Various conventional speed response type steering force control apparatuses for causing hydraulic reaction mechanisms to perform steering force control in correspondence with traveling speeds (vehicle speeds) of a vehicle have been proposed for power steering systems for increasing a steering wheel operating force (steering force) of a vehicle. More specifically, during parking or low-speed traveling, a reaction oil pressure is minimized to allow a steering operation with a small force. However, during high-speed traveling, the reaction oil pressure is increased to make the steering wheel stiff to improve stability during straight traveling. In this manner, steering force control is performed such that input and output shafts of the power steering system arc pivoted relatively to each other or constrained by a reaction piston for selectively constraining input and output shafts of the power steering system in accordance with a magnitude of the reaction oil pressure.
A typical steering force control apparatus of this type is disclosed in Japanese Patent Laid-Open Nos. 61-105273 and 61-132466, Japanese Utility Model Laid-Open No. 62-25265, and Japanese Patent Laid-Open No. 63-68467, wherein a reaction oil pressure is branched from part of a main hydraulic path extending from a pump to a power cylinder through a flow path selector valve, and this partial oil pressure is controlled by a reaction oil pressure control valve, e.g., a valve, and supplied to a hydraulic reaction chamber to move a reaction piston.
As described in the above prior art control apparatuses, a drive source for driving a control mechanism for controlling a reaction oil pressure transmitted to the hydraulic reaction chamber in accordance with the vehicle speed is generally constituted by electrical actuators such as a solenoid coil and a stepping motor for generating a predetermined operating force by using an output current from a controller in accordance with a detection signal from a vehicle speed sensor. In this speed response type steering force control apparatus of an electronic control system, however, the controller using a microcomputer or the like having a converter for converting the detection signal from the vehicle speed sensor into a current output is required, in addition, the electrical actuators such as a solenoid coil and a stepping motor driven by the output current are also required to inevitably result in high cost. Furthermore, a countermeasure against disturbance such as electromagnetic interference must be provided to assure stable control of the controller, thus further increasing the cost.
Another conventional steering force control apparatus of a hydraulic control system is also proposed as in Japanese Utility Model Publication No. 61-32851. This steering force control apparatus comprises a speed response type hydraulic pump whose delivery flow rate is changed in accordance with a change in vehicle speed, and a valve of a hydraulic control system having a spool which is operated in accordance with the delivery flow rate of the hydraulic pump to control a reaction oil pressure transmitted to a hydraulic reaction chamber. In this conventional apparatus, the spool is driven and controlled by increasing the delivery flow rate of the pump and utilizing a pressure difference between the upstream and downstream pressures of a stationary restrictor arranged in the path in the spool. At the same time, a reaction oil pressure delivered from the upstream side of the stationary restrictor is supplied to the hydraulic reaction chamber through a reaction path. A high flow rate is required to control and supply the oil pressure delivered from the speed response type pump to the hydraulic reaction chamber. As a result, a large pump is required, and the apparatus as a whole becomes bulky. Strong demand has arisen for providing an effective countermeasure for solving al the conventional problems.
In addition, in the steering force control apparatus using a hydraulic reaction mechanism of this type, the hydraulic reaction force must be proportionally controlled to have characteristics enough to obtain a steering force required in accordance with a vehicle speed and a steering condition. These problems are also taken into consideration.